1. Field of the Invention
The present invention relates to substrates for ink-jet printing. More particularly, the present invention relates to bank substrates for use in the deposition of ink-jet materials, such as those used in organic electronic devices such as organic transistors and integrated circuits, as well as for organic light-emitting devices (OLEDs).
2. Description of Related Art
One conventional type of printer forms characters and images on a medium or substrate, such as paper, by expelling droplets of ink, often comprising organic material, in a controlled fashion so that the droplets land on the medium in a pattern. Such a printer can be conceptualized as a mechanism for moving and placing the medium in a position such that ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate control hardware and software. A conventional print cartridge for an inkjet type printer comprises an ink containment device and a fingernail-sized apparatus, commonly known as a print head, which heats and expels ink droplets in a controlled fashion. The print cartridge may contain a storage vessel for ink, or the storage vessel may be separate from the print head. Other conventional inkjet type printers use piezo elements that can vary the ink chamber volume through use of the piezo-electric effect to expel ink droplets in a controlled fashion.
Applications of ink-jet printers have moved beyond the conventional creation of characters and images for viewing by people to the creation of circuits and displays, for example organic display panels.
Ink-jet printing is being used or developed as a tool to deposit organic materials in a patterned manner onto substrate to create organic, or partially organic, electronic devices such as, by example only, transistors (such as organic field-effect transistors) and integrated circuits, conductive via holes or traces, organic light-emitting devices (OLEDs). Helpful background material may be found in U.S. patent application Ser. No. 10/214,024, entitled “Drop Volume Measurement and Control for Ink Jet Printing”, filed on Aug. 7, 2002, and incorporated herein by reference.
The types of substrates used for ink-jet printing depends on the particular application. Some types of substrates used, by way of example only, are glass substrates, plastic substrates (such as polyethylene terephthalate, polyethylene naphthalate, polymide, polycarbonate), metal foils, ceramic substrates, laminated glass, and thin flexible glass. Some applications for substrates, by way of example only, are substrates for thin film transistors (TFTs), hybrid organic/inorganic TFTs, alpha-numeric or passive-matrix or active-matrix OLEDs or combined TFT/OLED devices.
A challenge with the techniques described above for creating printed film is maintaining the desired uniformity and quality of the resulting components, such as printed areas, printed segments, printed lines, printed pixels, printed traces, and printed via-holes, by way of example only.
The droplets striking the substrate often need to be contained in walls (such as troughs or banks, by way of example only) in or on the substrate to prevent spreading. However, excess fluid may collect near the walls, due to adhesion or simply because too many or too large droplets were deposited on the substrate. This excess fluid, or “flooding”, can mean that once the droplet(s) dries, ink remains beyond the boundary of the wall.
Additionally, droplets emitted by piezo-electric printing devices sometimes require more settling time, which means that the printing must start earlier. The misalignment and excess build-up of droplets can cause even more flooding. Excessive flooding may cause problems such as short circuits and current leaks; it may also make it more difficult to seal the device being manufactured.
For most applications, the solution should create a uniform film on the substrate which means, in most cases, that the solution must dry in a uniform manner as it is deposited. By way of example only, the uniformity of drying can affect, in the case of OLEDs, uniformity, lifetime, efficiency, and color gamut. Maintaining uniformity is difficult because very often solution drops show so-called “coffee stain” drying profiles. Capillary flow causes ring stains from dried liquid drops, whereby within a printed drop (or line or area) fluid-dynamic effects cause the edges of the drop (or line or area) to have a substantially higher concentration of solute than the center. This is often not acceptable for device performance. Though this effect can be mitigated by careful choice of the drying ambient (such as temperature and/or atmospheric pressure, by way of example only) and/or the choice of solvents (adjusting the boiling point, solvent strength, solvent mix, and/or surface energy, by way of example only), disadvantageously other printing factors are adversely affected substantially enough to degrade printing quality. For example, though coffee-stain types of drying effects can be reduced by using a highly viscous solvent and/or by using solute that is poorly dissolved in the solution (and therefore precipitates or “crashes out” of the solution), a highly-viscous solution or poorly dissolved solution can be very problematic for the printing or dispensing process itself (causing, by way of example only, nozzle clogging and/or inaccurate drop firing or dispensing).